As is well known, in a typical MOS transistor, source and drain of one conductivity type are formed in a body of opposite conductivity type. As devices continually shrink in size, in order for such devices to operate in the correct electrical mode, it is necessary that the depth of the source and drain in the body, i.e., the junction depth, be reduced also. For example, presently, with a polysilicon gate width of, for example, 0.25 .mu., junction depth should be on the order of 800.ANG..
Typically, in the prior art, achievement of such a small junction depth is problematical, in particular, for example with a p+ region formed using boron ions, as will now be described with regard to formation of said regions in a silicon crystalline layer.
In general, such boron ions are implanted with a chosen energy and a particular dosage necessary to control the concentration. The energy will determine the eventual depth of the junction. With boron being a very light element, in order to attempt to achieve a very shallow junction, it must be implanted at a very low energy. For example, 5 KeV is typically minimum energy for the boron implant.
It has been found, however, that during dopant activation anneal, boron diffusion in the crystalline silicon layer is significantly large, so that the junction depth of the boron tends to be much deeper than planned.
The problem of undefined dopant junction depth occurs because the implantation of the boron ions into the monocrystalline silicon layer causes implantation damage, in turn causing interstitial atoms of silicon to exist, i.e., atoms not in the crystal lattice but between lattice atoms. That is, silicon atoms are displaced from the monocrystalline lattice and are sitting between silicon atoms in the monocrystalline lattice. It has been found that during the high temperature step (also known as dopant activation anneal) described above to diffuse the boron into the monocrystalline silicon layer, boron diffuses by attaching to these interstitial silicon atoms, causing a very rapid diffusion of the boron into the monocrystalline silicon layer. Thus, typically, when boron is implanted into monocrystalline silicon and then an anneal step is undertaken, the dopant profile extends well beyond that desired, for example, X.sub.j depth of 2000.ANG. or more is formed. This is true even when implanting boron ions at a very low energy (i.e., .about.5 KeV).